Applicants have produced an improved long path length, temperature controlled, absorption cell designed to measure optical absorption of gas phase samples. Applicants' device is of the type known as a White-type absorption cell. These cells are based on optical configurations proposed by J. U. White in the J. Opt. Soc. Am., Vol. 32, (1942) pp. 285-288. The purpose of White-type absorption cells is to achieve long path lengths using multiple reflections and traversals within a small volume.
Many cells based on this theoretical proposal have been constructed, including a six meter base path length cell constructed by the Jet Propulsion Laboratory and located at the National Solar Observatory at Kitt Peak, Az. A White-type cell is commercially available from Laser Analytics Division of Spectra-Physics, Inc. In addition, a number of different designs have been recorded in the literature including W. R. Watkins and R. G. Dixon Rev. Sci. Instrum., Vol. 50, (1979), pp. 86-92: R. A. Briesmeister, G. W. Read, K. C. Kim and J. R. Fitzpatrick, Applied Spectroscopy, Vol. 38, (1984), pp. 35-38 and R. S. Eng and A. W. Mantz, J. of Mol. Spectry., Vol. 74, (1979), pp. 388-399. The disclosures of these publications are incorporated by reference herein.
Applicants' White-type cell permits very small absorption coefficients to be measured in condensable gases over a wide range of temperatures. Applicants' device has a long optical path and the cell maintains its stability over a wide range of temperatures and path lengths. Applicants' device has the mirror adjusting controls accessible on the exterior of the cell. The mirror spacing in applicants' device is stabilized so that the path length is constant over a very wide range of temperatures. Applicants' cell is useable over a wide range of temperatures, even with gas samples containing condensable fractions. Applicants' mirror surfaces are heated to prevent condensation of the sample on the mirror surfaces during use. Applicants also control the temperature of the entire absorption cell to permit elevated pressure (and high densities) of condensable samples to be used. In addition, applicants structure is incorporated within a small volume which permits much smaller samples of gas to be used, permitting use of a smaller and more homogeneous gas sample, thus increasing the accuracy of the measurements obtained.
The intensity of the light observed from a White-type cell following absorption by a gas sample depends upon the following equation: ln(I/I.sub.0)=-.sigma..sub.D nL where ln=the natural logarithm, I=the observed light intensity, I.sub.0 =the initial light intensity, .sigma..sub.D =the absorption cross section constant of the gas sample; n=the pressure and L=the path length. Applicants' device permits the pressure and/or the path length to be increased thus permitting increased accuracy in the measurements taken for any given gas, such as the value of .sigma..sub.D.
Applicants' device can be used in connection with a spectrophotometer to measure the composition of an unknown gas, for example, flue gases or stack gases, or to test the atmosphere for trace impurities and pollutants. The device can also be used to measure the fundamental properties of gases and gas mixtures of known composition. For example, to measure .sigma..sub.D by measuring the absorption of light in a gas sample of known composition as a function of the wavelength of light used.
It is thus an object of applicants' invention to produce a compact White-type cell capable of taking measurements of small, homogeneous gas samples.
It is a further object of applicants' invention to produce a White-type absorption cell in which the path length is stabilized over a wide range of operating temperatures.
It is a further object of applicants' invention to provide a White-type cell having a design which permits very small absorption coefficients to be measured in condensable gases.
It is an object of applicants' invention to produce a White-type cell which can function to measure condensable gases at elevated pressure.
It is an object of applicants' invention to provide a White-type cell having heated mirror surfaces to prevent condensation of gases on the mirror surfaces.
It is an object of applicants' invention to provide a White-type cell in which all mirror adjustment and controls are accessible from the exterior of the cell, permitting fine adjustment in use.
It is an object of applicants' invention to stabilize the mirrors and optical path on an Invar framework.
It is an object of applicants' invention to provide a temperature control for a White-type absorption cell and to provide means of controlling the temperature of the absorption cell during use.
It is a further object of applicants' invention to provide a White-type cell which can operate to measure corrosive gases and compositions.
Further objects and advantages of applicants' device will be apparent from the following description of the drawings and description of the preferred embodiments.